It got so hot on the "Dancing with the Stars" stage last night, one of the pros stripped to his Speedos -- and then it only got hotter!

The semi-finals of the All-Stars seasons went down on Monday, where Valentin Chmerkovskiy showed off a lot more than just his dance moves.

Maksim's brother put on a strip show during his "Surfer Flamenco" with partner Kelly Monaco ... totally stealing the show from his celebrity partner as he ripped off his pants to dance in nothing more than a black bathing suit.

Making it even sexier: The two danced in their swimsuits while on a mini stage filled with water, splashing the entire time.

But did the eye-catching action help them with the judges?

Not really... the dance only nabbed them a 28.5 with the judges, on a night full of perfect 30s.

It was one of two dances for the couple, who are now at the bottom of the leaderboard going into elimination night.